Method of making a calcium salt of 2-hydroxy-4-(mercaptomethyl) butanoic acid

ABSTRACT

A method of making calcium-2-hydroxy-4-(mercaptomethyl)butanoate includes combining and mixing 2-hydroxy-4-(mercaptomethyl)butanoic acid and a calcium salt selected from calcium oxide and calcium hydroxide and water in a first reaction zone. The combining and mixing step is carried out over a first period of 3-120 seconds. The resulting reaction mixture is transferred at the end of the first period from the first reaction zone to a second reaction zone, the transferring step being carried out over a second period of 3-60 seconds. Heat generated by reaction between the 2-hydroxy-4-(mercaptomethyl)butanoic acid and the calcium salt in the second reaction zone is allowed to drive off sufficient water to produce a product mixture containing less than 5% (m/m) water.

[0001] THIS INVENTION relates to the calcium salt of2-hydroxy-4-(mercaptomethyl)butanoic acid.

[0002] It relates in particular to a method of making the calcium saltof 2-hydroxy-4-(mercaptomethyl)butanoic acid and to the calcium salt of2-hydroxy-4-(mercaptomethyl)butanoic acid made in accordance with themethod. For brevity, the calcium salt of2-hydroxy-4-(mercaptomethyl)butanoic acid, which has the formula[CH₃S(CH₂)₂CH(OH)COO]₂Ca, is referred to hereinafter ascalcium-2-hydroxy-4-(mercaptomethyl) butanoate.

[0003] According to a first aspect of the invention, there is provided amethod of making calcium-2-hydroxy-4-(mercaptomethyl)butanoate, themethod including the steps of

[0004] combining and mixing 2-hydroxy-4-(mercaptomethyl)butanoic acidand a calcium salt selected from calcium oxide and calcium hydroxide andwater in a first reaction zone, the combining and mixing step beingcarried out over a first period of 3-120 seconds to produce a reactionmixture in the first reaction zone;

[0005] transferring the resulting reaction mixture at the end of thefirst period from the first reaction zone to a second reaction zone, thetransferring step being carried out over a second period of 3-60seconds; and

[0006] allowing heat generated by reaction between the2-hydroxy-4-(mercaptomethyl)butanoic acid and the calcium salt in thesecond reaction zone to drive off sufficient water to produce a productmixture containing less than about 5% (m/m) water.

[0007] The percentage (m/m) refers to a mass per mass percentage.

[0008] The first period may be 5-20 seconds. The second period may be5-45 seconds.

[0009] The 2-hydroxy-4-(mercaptomethyl)butanoic acid may be in the formof an aqueous solution. The aqueous solution may have a concentration of65-95% by mass of the 2-hydroxy-4-(mercaptomethyl)butanoic acid.Preferably it will have a concentration of 70-90% by mass. Typically, anapproximately 89% aqueous solution of2-hydroxy-4-(mercaptomethyl)butanoic acid is used. The compound isreferred to as methionine hydroxy analogue free acid and is commerciallyavailable. The method may include the prior step of mixing the acid andwater. The acid and the water may thus be kept in separate storagetanks.

[0010] The mass ratio between the aqueous acid solution and the calciumsalt will typically be between about 70-90% of the aqueous solution(expressed as an 88% aqueous solution), about 10-30% of the calcium saltand about 0-20% water (as added water) and will be affected by thepurity of the acid and the salt.

[0011] Thus the combining and mixing steps may be conducted so that thereaction mixture contains 70-90% by mass of the aqueous solution, 10-30%by mass of the calcium salt and 0-20% by mass water.

[0012] Preferably, the method will include allowing the heat generatedto drive off sufficient water to produce a product mixture containingless than about 4.5% water and more preferably less than about 2%.

[0013] The combining step may include adding the calcium salt to thesolution of 2-hydroxy-4-(mercaptomethyl)butanoic acid. The method mayinclude the prior step of warming the solution of2-hydroxy-4-(mercaptomethyl)butanoic acid to about 50-130° C. andpreferably to about 70-85° C.

[0014] The duration of the combining and mixing step will vary with theamounts of the acid and calcium salt and with the nature of the calciumsalt. For example, in the case of calcium oxide, the time to combine theacid and the calcium salt will vary from a lower level of about 3-10seconds for an amount of about 55-65 kg of the acid and preferably about60 kg of the acid and about 9-18 kg of the calcium oxide/hydroxide to anupper level of about 3-30 seconds for an amount of about 500 kg of theacid and about 75-150 kg of the calcium oxide/hydroxide. As thedischarge of any water required can be done simultaneously with theacid, no extension of the charging time will be brought about by wateraddition.

[0015] The duration of the second period will vary similarly. In thecase of calcium oxide the duration of the second period will vary from alower level of about 3-15 seconds for an amount of about 55-65 kg of theacid and preferably about 60 kg of the acid and about 9-18 kg of thecalcium oxide/hydroxide to an upper level of about 3-20 seconds for anamount of about 450-550 kg of the acid and preferably about 500 kg ofthe acid and about 75-150 kg of the calcium oxide/hydroxide. Theaddition of water will generally shorten the second period.

[0016] Thus the salt may be calcium oxide and 55-65 kg of the aqueoussolution may be combined with 9-18 kg of the calcium salt over a firstperiod of about 3-10 seconds. The duration of the second period may thenbe about 3-15 seconds.

[0017] Instead, the salt may be calcium oxide and about 450-550 kg ofthe aqueous solution may be combined with about 75-150 kg of the calciumsalt over a first period of 3-30 seconds. The duration of the secondperiod may then be about 3-20 seconds.

[0018] The method may include the further steps of successivelycombining and mixing a plurality of batches of the acid, calcium saltand water in the first reaction zone to produce successive batches ofthe reaction mixture and successively transferring each of the batchesto the same second reaction zone.

[0019] The second reaction zone may be a receiving vessel and severalbatches of reaction mixture may be transferred into the same receivingvessel until the receiving vessel is full or contains a predeterminedquantity of the reaction mixture. The receiving vessel may then bereplaced with a second receiving vessel and further batches may be addedto the second receiving vessel. This process may be continued until theinitially used receiving vessels have been emptied so that they can bereused. It is an advantage of this embodiment of the invention that thereaction vessel and all of the receiving vessels can be kept within anenclosed area so that steam and fumes produced in the reaction betweenthe acid and the calcium salt can be extracted and dealt with by anappropriate disposal system.

[0020] In an example of this embodiment of the invention, a materialdosing system, controlled by a programmable logic controller or similarsuitable control unit, weighs out the raw materials in sequence into amixing vessel. The agitator in the mixing vessel starts immediately onreceiving a signal that the first of the pre-heated acid has been dosed.Optionally there can be an operator-controlled delay between the openingof the acid dosing valves and the beginning of the agitation.

[0021] Thus the method may be one in which a material dosing systemcontrolled by a programmable logic controller, weighs out the rawmaterials into the first reaction zone.

[0022] Once agitation has begun, the calcium salt is dosed. The rawmaterials are then mixed for a specific time before being dischargedinto a receiving vessel. Additional batches are then dosed into the samereaction vessel and discharged into the receiving vessel until theamount of material in the receiving vessel has reached a desired level.Optionally, the material in the receiving vessel can also be agitated.Once the desired level in the receiving vessel has been reached, it isremoved from the dosing area and replaced by a fresh receiving vessel.The process then continues with reaction mixtures being charged into thefresh receiving vessel. The first receiving vessel can then be moved toanother location whilst the reaction proceeds to completion with theheat produced driving off water. There is essentially no limit to thenumber of receiving vessels that can be employed and the size of thereaction vessels can be selected to suit the residence time required toensure complete reaction and a desired output.

[0023] In another example of this embodiment of the invention, usingcalcium oxide as the calcium salt, the required amount of a pre-warmedsolution of 2-hydroxy-4-(mercaptomethyl)butanoic acid is added to amixing vessel fitted with an agitator and the required amount ofpre-weighed calcium oxide is rapidly added to the solution undervigorous agitation. The addition typically takes about 3-10 secondsdepending upon the volume of the solutions and the mass of the calciumoxide. Reaction between the acid in the solution and the calcium oxideusually starts within 5-30 seconds, depending upon the reactivity of thecalcium oxide, the temperature of the acid solution and theconcentration of the acid, and is accompanied by a rapid temperaturerise to about 120° C. within 5-30 seconds for small batches. Thetemperature rise in larger batches is usually less rapid, taking up to10 minutes to rise to 120° C. The mixture is immediately discharged intoa receiving vessel or onto a flat surface. These process steps arerepeated several times and the several resulting batches are dischargedon top of one another. The number of batches is determined by the sizeof the mixing vessel and the rate of loss of water from the reactingmagma. When the receiving vessel is full, further batches are dischargedinto a fresh receiving vessel. Once the material in the first receivingvessel is dry enough (i.e. typically having a water content of less thanabout 10% and usually less than about 5%), the contents are dischargedand the vessel is reused. The contents of the first receiving vessel aregenerally discharged when the second receiving vessel is full and thecontents of the second vessel are discharged when the first vessel isfull. The discharged material is allowed to cool with occasionalagitation to allow heat to escape. The final product is then dressedaccording to size and packaged.

[0024] Preferably, the reaction vessels will be designed to have nosharp internal corners or edges. The vessels will also preferably belined with an inert material such as a layer of PTFE(polytetrafluoroethylene) or a similar material. The top of themixing/reaction vessel will preferably be provided with an extractionsystem for extracting of steam and vapours. The inner lining materialserves to reduce adhesion between the reactants, the product and thevessel to facilitate rapid discharge when the vessel is tipped.

[0025] According to a second aspect of the invention, there is provideda method of making calcium-2-hydroxy-4-(mercaptomethyl)butanoate, themethod including the steps of

[0026] combining and mixing 2-hydroxy-4-(mercaptomethyl)butanoic acid, acalcium salt selected from calcium oxide and calcium hydroxide and waterin a first reaction zone to produce a reaction mixture in the firstreaction zone; and

[0027] continuously transferring the reaction mixture from the firstreaction zone to a second reaction zone, the reactants being added tothe first reaction zone in successive batches and the reaction mixturebeing continuously removed from the first reaction zone at a rate whichis selected so that the residence time of the reaction mixture in thefirst reaction zone is between about 3 and 120 seconds; and

[0028] allowing heat generated by reaction between the2-hydroxy-4-(mercaptomethyl)butanoic acid and the calcium salt in thesecond reaction zone to drive off sufficient water to produce a productmixture containing less than about 10% water.

[0029] Preferably the method will involve allowing heat generated todrive off sufficient water to produce a product mixture containing lessthan about 5% water.

[0030] The rate of removal of the reaction mixture from the firstreaction zone will depend on the production capacity of the installationand may vary between about 500 and about 30000 kg per hour.

[0031] According to another aspect of the invention, there is provided acontinuous method of makingcalcium-2-hydroxy-4-(mercaptomethyl)butanoate, the method including thesteps of

[0032] simultaneously feeding, into a reaction zone, an aqueous solutionof 2-hydroxy-4-(mercaptomethyl)butanoic acid, at a rate of approximately500-30000 parts per hour, and a calcium salt selected from calcium oxideand calcium hydroxide, at a rate of approximately 75-6200 parts perhour, to produce a reaction mixture in the reaction zone; and

[0033] transferring the reaction mixture from the reaction zone to asecond zone at a rate which is selected so that the residence time ofthe reaction mixture in the reaction zone is sufficient to allow heatgenerated by reaction between the 2-hydroxy-4-(mercaptomethyl)butanoicacid and the calcium salt in the reaction zone to drive off sufficientwater to produce a product mixture containing less than about 5% water.

[0034] The rate will preferably be selected so that the residence timeof the reaction mixture in the reaction zone is 5-30 minutes.

[0035] In an example of this embodiment of the invention, a pre-warmedsolution of 2-hydroxy-4-(mercaptomethyl)butanoic acid, at a temperatureof about 50-130° C. and preferably at a temperature of about 70-85° C.,is transferred at a controlled rate into a mixing chamber. Theincorporation of water would change these temperatures. Calcium oxide issimultaneously added to the mixing chamber at a controlled rate. At thestart of the continuous process the outlet from the mixing chamber iskept closed. This allows reacting material to build up in the mixingchamber and the temperature to rise so that the chamber is effectivelyboth a mixing and a reaction zone. The temperature of the material inthe chamber rapidly rises to about 100-170° C. with the evolution ofwater as steam. Once the initial reaction is over and a powdery producthas formed, addition of the warm solution and the calcium oxide isresumed. The rate of addition of the solution and the calcium oxide isselected so that the residence time in the vessel is sufficient to allowthe 2-hydroxy-4-(mercaptomethyl)butanoic acid and the calcium oxide toreact and for water to be driven off. The residence time is typicallyabout 5-30 minutes. Addition of the solution and the calcium oxide thencontinues and the outlet from the mixing chamber is opened so that thereaction product is discharged. The amount of product remaining in thechamber is controlled at all times to ensure that a suitably dry productis continuously discharged from the chamber at a controlled rate. Thereaction product is discharged onto a surface or into a den and left tocool or taken for further processing as required.

[0036] According to another aspect of the invention, there is provided acontinuous method of makingcalcium-2-hydroxy-4-(mercaptomethyl)butanoate, the method including thesteps of

[0037] simultaneously feeding, into a reaction zone, an aqueous solutionof 2-hydroxy-4-(mercaptomethyl)butanoic acid, at a rate of approximately500-30000 parts per hour, and a calcium salt selected from calcium oxideand calcium hydroxide, at a rate of approximately 75-6200 parts perhour, to produce a reaction mixture in the reaction zone; and

[0038] transferring the reaction mixture from the reaction zone to asecond zone at a rate which is selected so that the residence time ofthe reaction mixture in the reaction zone is sufficient to initiatereaction between the 2-hydroxy-4-(mercaptomethyl)butanoic acid and thecalcium salt but not sufficient to drive off water from the reactionmixture and allowing heat generated by further reaction between the2-hydroxy-4-(mercaptomethyl)butanoic acid and the calcium salt in thesecond zone to drive off sufficient water to produce a product mixturecontaining less than about 5% water.

[0039] The rate of addition of the acid and the salt will depend uponthe production capacity of the plant in which the process is conducted.Production levels of 1-30 tons per hour can be achieved by the method ofthe invention.

[0040] In an example of this embodiment of the invention, the amount ofmaterial retained in the mixing chamber is less and the mixing chamberserves only to mix the two reactants and to allow reaction to initiate.In this case the residence time in the mixing zone is about 5-60seconds. Further reaction and water loss from the reaction mixture thencontinues after the mixture has been discharged from the mixing vessel.The product is then screened or packaged as described above.

[0041] The reaction mixture in the mixing/reaction zone may betransferred on an endless moving belt so that the second zone is formedby the belt. The reactants will then react whilst being carried on thebelt. The length of the belt and the speed of the belt will be selectedso that, when the material is discharged from the belt, the resultingproduct mixture contains the desired amount of water. The belt willpreferably be enclosed so that steam and fumes produced by reactionbetween the acid and the calcium salt can be extracted and dealt with byan appropriate disposal system as described above.

[0042] Preferably, at the point of discharge from the belt, the materialwill be passed through a rotating cutter, to reduce the particle sizeand to release trapped steam and gases in the product. The product willthen be transported on a conveyor or suitable handling device forfurther processing as required, for example for granulation, drying,cooling, dressing or bagging.

[0043] There are several important advantages associated with thisembodiment of the invention. Because there is minimal mixing of theproduct after the initial feeding and mixing stage, energy requirementsof the process are reduced. Furthermore, when the product passes throughthe plastic stage, no mechanical agitation takes place. There istherefore no contact between a mixing device and the thickening productand no build-up or aggregation of material in the mixing vessel or onthe agitator. It has been found that this largely removes therequirement of high pressure steam cleaning of equipment and theresulting effluent problem. It is also an advantage that the entirereaction system is enclosed. This allows relatively easy removal ofsteam, water vapour and gases produced in the reaction. The Applicanthas also found that there is no need for recycle or heel material. Thiseffectively increases the production throughput for equipment of a givensize. Prior art methods known to the Applicant require the addition ofrecycle or heel material.

[0044] In an example of this embodiment of the invention, the hotaqueous acid solution is weighed out into the mixing/reaction vessel.The agitator is started and the calcium oxide or calcium hydroxide isdosed. After a predetermined period, the outlet valve of the mixingvessel opens and the contents discharge onto an endless enclosed beltrunning, typically, between two pulleys.

[0045] In this embodiment of the invention an agitator can be includedat the beginning of the belt close to the discharge from themixing/reaction vessel to ensure that complete mixing takes place beforethe material passes from the fluid stage. Preferably, the initial partof the belt has a U-shaped cross-section in order to hold the relativelyfluid reaction mixture. The initial part of the belt is also arranged toslope downwardly from the reaction vessel to prevent reverse flow orspillage of material. The U-shaped section is selected to have a volumewhich is sufficient to hold up to 30 minutes' of plant productioncapacity. During this period, the reaction mixture passes through afluid and then a plastic stage with the evolution of steam and gases. Atthe end of the U-shaped section, the belt flattens out to almost itsfull width but remains slightly curled at the outside edges to minimizethe risk of spillage. At the point at which the belt flattens out, theproduct has already partially dried. The flattening of the belt causesthe cake to split open to release steam, moisture and gases trappedinside the cake. The thickness of the cake on the belt generally variesbetween about 10 and about 90 cm. The cake is then carried by the belt,drying as it moves and, at the end of the belt, the cake falls into acrumbling device. This reduces the particle size of the product andreleases steam, moisture and trapped gases. The product is thentransported to a second locality for treatment such as granulation,drying, cooling, enrichment, sizing or bagging.

[0046] In a preferred embodiment, the entire belt is enclosed by acanopy. Air is drawn through the enclosed space carrying with it steam,water vapour, fumes and gases produced in the reaction. The extractedgases are treated in a suitable treatment plant.

[0047] Preferably the belt will be made of a suitable high temperatureresistant material, PTFE, thin stainless steel, wooden slats or thelike. The release-nature of the belt is also important. The belt must besufficiently smooth to allow the product to drop off the belt as itpasses around the final pulley. Ideally, no material should adhere tothe belt. The enclosed belt system will typically have a width of about2 m a length of approximately 25 m and a maximum height of about 2 m.

[0048] In other embodiments of the invention, the calcium salt isreplaced with a metal salt selected from magnesium oxide, sodiumhydroxide and potassium hydroxide.

[0049] Thus, according to another aspect of the invention there isprovided a method of making a salt of2-hydroxy-4-(mercaptomethyl)butanoate, the salt being a salt of a metalselected from magnesium, sodium and potassium and the method includingthe steps of

[0050] combining and mixing 2-hydroxy-4-(mercaptomethyl)butanoic acidand a base selected from magnesium oxide, sodium hydroxide and potassiumhydroxide in water in a first reaction zone, the combining and mixingstep being carried out over a first period of 3-120 seconds to produce areaction mixture in the first reaction zone;

[0051] transferring the resulting reaction mixture at the end of thefirst period from the first reaction zone to a second reaction zone, thetransferring step being carried out over a second period of 3-60seconds; and

[0052] allowing heat generated by reaction between the2-hydroxy-4-(mercaptomethyl)butanoic acid and the base in the secondreaction zone to drive off sufficient water to produce a product mixturecontaining less than 5% (m/m) water.

[0053] The invention extends to a magnesium, sodium or potassium salt of2-hydroxy-4-(mercaptomethyl)butanoate prepared by a method ashereinbefore described.

[0054] Where the raw materials, or reactants, are dosed on a batchbasis, the dosage can be by weight or volume. Where the reactants aredosed on a continuous basis, the amount dosed will be measured by a beltweigher, mass flow meter or a similar dosing device.

[0055] The invention is now described, by way of example, with referenceto the following Examples and the drawings in which

[0056]FIG. 1 shows a schematic plan view of an installation for use inthe method of the invention;

[0057]FIG. 2 shows a schematic side view of part of the installation ofFIG. 1 with a hopper in an upright position;

[0058]FIG. 3 shows the side view of FIG. 2 with the hopper in a tiltedposition; and

[0059]FIG. 4 shows a schematic side view of another installation for usein the method of the invention.

EXAMPLE 1 Batch Process

[0060] 2-Hydroxy-4-(mercaptomethyl)butanoic acid (89% aqueous solution;1008 g; specific gravity 1.24) was heated to 84° C. in a 2.5 l vesseland calcium oxide (177 g) was rapidly added to the heated liquid. Thecalcium oxide contained 93% as CaO and 0.5% as MgO. The addition tookless than about 5 seconds. The resulting mixture was thoroughly stirredfor 15 seconds and then poured into a receiving vessel and left to standwithout any further stirring. The mixing, stirring and pouring stepstook less than about 30 seconds. The temperature of the mixture roserapidly to about 90° C. in about 60 seconds and then continued to riseover a period of about 25 minutes to a maximum of about 139° C. The massof the mixture was monitored for 40 minutes after it had been pouredinto the receiving vessel to measure water loss due to evaporation fromthe hot mixture.

[0061] The time, temperature and mass are set out below: TIME(min)TEMPERATURE(° C.) MASS(g) 15 134 1106 20 137 1101 25 139 1096 30 1361094 40 126 1093

[0062] The reaction mixture was then turned over several times to allowadditional moisture to escape. The final mass was 1050 g. The mixturewas screened into two fractions which were designated as “coarse”(particle size 4.00-0.5 mm; 653 g) and “fine” (particle size less than0.5 mm; 380 g). Approximately 17 g of material was lost during thescreening process. Each fraction was analysed for monomer acid content.The “coarse” fraction contained 86.5% and the “fine” fraction 83.9% ofthe monomer acid. The above values correspond to 97.4% for the “coarse”and 94.5% for the “fine” expressed ascalcium-2-hydroxy-4-(mercaptomethyl)butanoate.

EXAMPLE 2 Batch Process

[0063] 2-Hydroxy-4-(mercaptomethyl)butanoic acid (89% aqueous solution;10.00 kg) was heated to 73° C. and calcium oxide (1,768 kg) was rapidlyadded to the heated liquid with agitation. The calcium oxide contained93% as CaO and 0.5% as MgO. The addition took about 8 seconds. Theresulting mixture was thoroughly stirred for about 3 minutes, using anelectric drill fitted with a blade resembling a kitchen whisk, duringwhich time the temperature initially dropped to 63° C. and then rose to90° C. The mixture was then poured into a receiving vessel and left tostand without any further stirring. The mixing, stirring and pouringsteps took about 4 minutes. The temperature of the mixture rose rapidlyto boiling point (about 121° C.) in about 7 minutes. A second batch of2-hydroxy-4-(mercaptomethyl)butanoic acid (89% aqueous solution; 10.00kg) and calcium oxide (1.768 kg) was mixed in the same way and added tothe same receiving vessel and the combined mixture was left for 90minutes and weighed. The water loss after 90 minutes was found to be2246 g. The weight was 21290 g. The product was then poured out onto aflat tray. After 16 hours the product was re-weighed and the water losswas found to be 2861 g. It was then screened, sampled and analysed. Themonomer acid content of the product was 85.1% corresponding to 95.8%expressed as calcium-2-hydroxy-4-(mercaptomethyl) butanoate.

EXAMPLE 3 Batch Process

[0064] Successive batches of 2-hydroxy-4-(mercaptomethyl)butanoic acid(89% aqueous solution; specific gravity 1.24) and calcium oxide(unslaked lime, CaO content 93%, MgO content 0.5%) were mixed and, ineach case, agitated until the temperature reached about 80-90° C. Thebatches were then successively dropped into a 50 l capacity container.The maximum temperature reached in the container was approximately 219°C. After 40 minutes, the resulting product was dry and free-flowing. Thebed depth in the container was 15 cm. After 16 hours even the stickymoist product which had adhered to the top and edges of the containerhad dried. LIQUID HMBA CaO TEMPERATURE BATCH NO. (g) (g) (° C.) 1 1004177 74 2 1008 177 70 3 1003 177 72 4 1004 180 69 5 1005 178 63 6 1002178 70 TOTAL 6026 1067 

[0065] The product analysed at 85.6% monomer acid content, 13.1% Ca and2.3% free moisture.

EXAMPLE 4 Batch Process

[0066] 2-Hydroxy-4-(mercaptomethyl)butanoic acid (89% aqueous solution;100 g) was heated to 77° C. and calcium oxide (18.8 g) was rapidly addedto the heated solution with agitation. The calcium oxide contained 90%as CaO and 2.3% as MgO. The mixture was intensively mixed by hand in alarge container and the temperature rapidly rose to 123° C. The productwent through a plastic stage and then broke up into a free-flowingpowder. The yield was 104.5 g of the calcium salt with an analysis of85.29% monomer acid content and 2.1% free moisture.

EXAMPLE 5 Totally Enclosed Batch Process

[0067] Referring to the drawings, reference numeral 10 generallyindicates an installation for the continuous production of calcium2-hydroxy-4-(mercaptomethyl)butanoate. The installation 10 includes acarousel 12 which rotates in the direction of the arrows 14. Twelvehoppers 16.1-16.12 are mounted on the carousel 12.

[0068] The installation 10 includes a calcium oxide silo 18 and astorage tank 20 which holds a preheated 88% aqueous solution of2-hydroxy-4-(mercaptomethyl)butanoic acid. Feed lines 22, 24 extend fromthe silo 18 and the tank 20 to a mixing installation 26 which includes amixing vessel (not shown) and a measuring device (not shown) formeasuring the volume or mass of the calcium oxide and the aqueous acid.A feed line 28 extends from the mixing installation 26 for feeding thereaction mixture into the hoppers 16.1-16.12.

[0069]FIGS. 2 and 3 show the hopper 16.12 and a part of the carousel 12in further detail. The carousel 12 includes a rotatable support platform30 on which the hoppers 16.1-16.12 are mounted on pivot mechanismsgenerally indicated by reference numeral 32 so that each hopper can betilted in the direction of the arrow 34 as shown in FIG. 3. A hydraulicarm, (not shown) operates to tip the hoppers so that the reactionmixture can be discharged. The installation 10 further includes aspillage plate 56.

[0070] The carousel 12 and the hoppers 16.1-16.12 are enclosed in anannular tunnel structure 40 (FIGS. 2 and 3) which is provided with anextractor 42 (FIG. 1). The tunnel structure 40 is provided with a door46 which opens in the direction of the arrow 48 by a hydraulic rammechanisms 50 when a hopper 16.1-16.12 is tipped as shown in FIG. 3.When the hopper 16.1-16.12 returns to its upright position on thecarousel 12, the door 46 closes again in the direction of the arrow 52.Material 51 tipped from the hopper is optionally passed through thecrumbler (not shown) and falls onto a conveyor belt 54 from where it isconveyed to a receiving vessel.

[0071] In this embodiment of the invention, calcium oxide and the acidare mixed in batches to produce 62.5 kg of product in the mixing vesselfor up to 45 seconds for each batch and the reaction mixture is thentransferred into one of the hoppers 16.1-16.12 over a period ofapproximately 3-10 seconds. About 16 batches are added to each of thehoppers 16.1-16.12 so that each hopper holds approximately 1 metric tonof the reaction mixture. Each hopper 16.1-16.12 is accordingly filled inabout 12 minutes. When a hopper is full, the carousel 12 rotates so thatthe next hopper is positioned below the feed line 28 to be filled. Tenof the hoppers are filled over a period of 2 hours. When the firsthopper reaches a position opposite the conveyor belt 54, correspondingto hopper 16.12 the hydraulic arm tips the hopper so that the reactionmixture, which has by then been reacting for up to 2 hours, is tipped(optionally via the crumbler) onto the conveyer belt 54. In thisembodiment of the invention, the installation will produce approximately5 tons of product per hour.

[0072] In another embodiment of this version of the invention, thehoppers 16.1-16.12 are larger and the aqueous acid and the calcium oxideare combined in batches, as described above, at a rate of approximately5.8 tons per hour of the aqueous acid and 1.13 tons per hour of thecalcium oxide to produce about 6 tons per hour of product. The actualamounts of the aqueous solution, the water and the calcium salt dosedand the time that the reactants remain in the hoppers will depend uponthe desired throughput and the purity of the raw materials. The rates ofaddition and the quantities will be adjusted accordingly.

[0073] In other embodiments of this version of the invention, the sizesof the hoppers 16.1-16.12 are varied and the rate of addition of thecalcium oxide is accordingly varied between about 315 and about 6200 kgper hour and the rate of addition of the aqueous acid is varied betweenabout 1 500 and about 30 000 kg per hour. The method of the inventioncan thus produce as much as 30 tons per hour, or more, of the calciumsalt.

EXAMPLE 6 Open Process

[0074] Referring now to FIG. 4, reference numeral 80 generally indicatesanother embodiment of an installation for the production of calcium2-hydroxy-4-(mercaptomethyl)butanoic. The installation 80 includes apair of reaction vessels or bins 82, 84 which are pivotally mountedwithin a housing, generally indicated by reference numeral 86. A part ofthe housing 86 forms a roof 88 above the bin 82 and a roof 90 above thebin 84. A feed conduit 92 extends from a reaction vessel 96. The conduit92 can swivel in the direction of the arrow 94 so that it can feedreaction mixture from the reaction vessel 96 through the roofs 88, 90into the bins 82, 84. Extraction conduits 97, 98 extend from the roofs88, 90 to a common extraction conduit 100.

[0075] The bins 82, 84 are pivotally mounted so that they can be tipped(the bin 82 being depicted in its tipped position). Stop formations 102,103 are provided below the bins 82, 84 and are positioned so that impactof the bin 82, 84 on the stop formation 102, 104 will serve to dislodgematerial in the bin 82, 84. Material 104 tipped from the bins 82, 84falls onto a conveyor 106. In another embodiment of the invention (notshown), the stop formations 102, 104 are replaced with cam mechanismswhich jolt the bins to dislodge the contents so that an entire load (upto 2 tons of material) is not dislodged in a single mass.

[0076] In the embodiment depicted in the drawings the bins are linedwith polytetrafluoroethylene or a material having similar properties andhave a capacity of about 3 m². The bins 82, 84 are approximately 1.9 min diameter with a cylindrical section of about 0.5 m and a 1.9 mdiameter hemispherical bottom. In the filling position as shown for bin84, the bin 84 is substantially flush against the roof 90 above it. Inanother embodiment of the invention (not shown) the reactants are fedinto the bin and fumes are removed from the bin via a common conduit.The installation 80 also includes mechanisms (not shown) for tipping thebins 82, 84 and for returning them to their untipped positions. Theinstallation 80 also includes valves (not shown) in the extractionconduits 96, 98 which are closed by the PLC when the dropping mechanismis activated to prevent air being pulled through the relevant side ofthe extraction system. Each valve is reopened when the mechanism returnsthe bin to its upright position. In this embodiment of the invention,the PLC ensures that one bin is always in the filling position thusminimizing risk of damage to the extraction fan. The surge bin itself ispartially closed off for example with small suctions in the roof (notshown) or via the enclosed belt conveyor.

[0077] The bins 82, 84 are fed via a revolving chute (not shown) underthe exit of the reactor vessel 96. The chute swivels to feed bin 82 or84 as required. This embodiment of the invention does not require avalve at the bin as the reactant flow is controlled from the reactor andmaterial only flows down the chute when the bin is in its uprightposition. The inlet not in use permits air to be sucked through from theoutside thereby minimizing odours in the general area.

[0078] This particular embodiment of the invention is relativelyinexpensive. There are few moving parts, no operator exposure and theentire installation is enclosed.

Discussion

[0079] Calcium-2-hydroxy-4-(mercaptomethyl)butanoate is the α-hydroxyanalogue of the calcium salt of methionine and is an important feedsupplement, particularly for poultry. It is accordingly an importantproduct in the animal feed industry.

[0080] The invention is based on the reaction between the acidic2-hydroxy-4-(mercaptomethyl)butanoic acid and the basic calcium oxide.Calcium hydroxide may also be used but the reaction produces moremoisture. The acid is in the form of an aqueous solution and the totalmoisture which has to be removed is the moisture arising from thesolution and that produced in the reaction. The moisture content of thefinal product is preferably less than about 2% (m/m). The calcium oxideused in the process is not necessarily chemically pure and may containlimited quantities of calcium hydroxide and calcium carbonate. Theamount of these impurities present influences the amount of heatgenerated. In addition, impurities in the calcium oxide such asmagnesium oxide, iron and aluminium salts, sulphates and silicacompounds can also influence the reaction. The physical nature and theway in which the calcium oxide is processed can also affect thereaction. It is accordingly difficult to predict whether or not aparticular sample of calcium oxide will react satisfactorily to producethe desired free-flowing product. In those cases where the exothermicityof the reaction is insufficient to remove the required amount of water,a drying step can be introduced prior to storage or bagging.

[0081] There are effectively three ways in which the reaction can becarried out. These are referred to below as the “open process”, the“totally enclosed batch process” and the “enclosed continuous process”.All three processes are based on controlling the amount of acid in theform of a warm or hot aqueous solution of2-hydroxy-4-(mercaptomethyl)butanoic acid and base in the form ofcalcium oxide or hydroxide used.

The Open Process

[0082] The open process is a continual batching operation. In thisprocess, the combination of the raw materials and the frequency ofbatches is fast enough to match that of a continuous process even thougheach raw material batch is weighed individually. It is generallynecessary to maintain the acid solution at a temperature of about 80-85°C. and, to achieve this, a separate heating tank is usually operatedseparately from the acid storage tank. As hot acid solution is drawnfrom the heating tank for use in the process, it is replenished by freshacid solution via a heat exchanger. Alternatively, the heating tank canbe fitted with internal or external heating coils.

[0083] The success of the process is determined by rapid and continuousbatching of the raw materials, rapid mixing of the raw materials andrapid transfer of the reacting reaction mixture, or magma.

[0084] Material Dosing

[0085] In the dosing stage each of the raw materials i.e. water, thecalcium oxide and the hot aqueous solution of the acid are measuredseparately but simultaneously and precise quantities are dischargedsequentially into the mixing vessel. The operation is controlled by aprogrammable logic controller (or PLC) to ensure repeatability andaccuracy of both the quantities and time intervals involved.

[0086] As soon as the acid or water tanks or the unslaked lime hopperwhich form parts of the mixing installation have been emptied, the PLCinitiates the weighing sequence and refills the vessels with the correctquantities of material from storage. Thus, by the time the reactionmixture is ready for discharge from the reactor, the raw materials forthe next batch have already been weighed out. Typically the dischargingof the raw material takes about 3-10 seconds. However this may besignificantly longer in the event of large quantities of lime with poorflow characteristics.

[0087] Mixing

[0088] The raw materials are discharged in sequence into the reactorwith the acid discharged first followed by any formulation water. Assoon as the valve opens to discharge the acid, the agitator starts. Oncethe acid valve has opened and the agitator has started, the PLC willdispense the water and open the lime discharge valve. After a presetinterval, compressed air is blown into the lime weighing vessel to forcethe unslaked lime out. Thus the acid, water and lime are dischargedalmost simultaneously into the mixing zone created by the rotatingagitator.

[0089] Mixing is continued for a predetermined time, usually betweenabout 5 and 30 seconds. This time is determined by how rapidly thereaction mixture begins to boil. The mixing time can be longer in thecase of a coarse or lower reactivity lime or where the acid is at alower temperature than normal.

[0090] The mixer is linked to a fume extraction system to remove steamand fumes produced during the reaction.

[0091] The Rapid Transfer Stage

[0092] Initially, a liquid mixture comprising free acid, unslaked lime,water and some calcium-2-hydroxy-4-(mercaptomethyl)butanoate is presentin the reactor. If this mixture is left in the reactor, the temperaturerises rapidly, and the reaction mixture or magma starts to lose waterand becomes extremely plastic and sticky. Ultimately this material wouldadhere to the mixing blades and the sidewall and transferring thematerial would be very difficult.

[0093] Before this happens, and after a preset time, the PLC opens thedischarge valve of the reactor so that the reaction mixture is rapidlytransferred into an adjacent unstirred reaction zone or den. The objectof the rapid mixing step and rapid transfer is to initiate but not tocomplete the reaction between the acid and the unslaked lime so that thereaction is completed in a separate reaction zone.

[0094] After a preset time, the discharge valve on the reactor closes,and the entire cycle begins again. However, during the mixing andtransfer periods, the acid, water and lime weighing vessels have beenrefilled with the desired quantity of raw materials. There isaccordingly no significant time delay waiting for the weighing stages tobe completed. The reacting magma usually takes approximately 3-15seconds to discharge.

[0095] Final Reaction Stage

[0096] In the reaction zone, the reaction is allowed to go tocompletion. However in order to fully utilize the heat of reactionbetween the acid and the lime, several batches are dropped sequentiallyinto the same reaction zone or den. Most of the steam and fumes arisingfrom the reaction, occurs in this zone and accordingly every reactionzone and den is fitted with suitable fume and steam extractionequipment.

[0097] Once a den, or reaction zone, is full the reactor outlet swivelsand commences discharging into a second (or third) adjacent den. Thesame sequence is followed until the second den is full. Whilst one denis filling up, the other(s) are matured or emptied. Generally thematerial remains in the static den for a period ranging from 15 to 60minutes.

[0098] The material removed from the den is generally transported to abay to mature and cool, or taken to further processing stages such asgranulation, enrichment, cooling and dressing prior to packaging.

[0099] Typically one full batch cycle, which produces approximately 250kg of the calcium salt product takes about 25-45 seconds.

[0100] Drying

[0101] It has been found that the initial product discharged from thedens has a moisture-content of about 3-5% and already contains at least83% of the monomer acid. If this product is left to mature and cool in abay, the mixture content drops to below 2% over a period of about aweek. With the moisture loss, the monomer acid content rises typicallyto over 85%.

[0102] It is not desirable to leave the product in a heap for anextended period of time. Therefore the material is generally run througha drying stage. This results in the moisture content dropping to lessthan 2.0%, and as low as 0.5%, with a corresponding increase in monomeracid-content to above 85%. This is equivalent to over 96% expressed asthe calcium salt.

The Totally Enclosed Batch Process

[0103] There are several differences between the “open process” and the“totally enclosed batch process”.

[0104] In the “open process” the reacting magma is discharged into oneor more static or moveable reaction zones or dens. These dens are fittedwith fume extraction apparatus to remove the fumes and steam from thereaction area.

[0105] In the “totally enclosed batch process”, there are a number ofreaction dens on a carousel as described in Example 5 above. However allthe dens are located within a single totally enclosed fume and steamextraction system. The raw material weighing system and reactor areusually separated from and positioned above position number 1 of thecarousel. However fumes and steam arising in the weighers or in thereactor are also withdrawn to the fume extraction system.

[0106] Material is weighed out and discharged in a manner similar to thebatch system. The charging time of the den is very similar to that ofthe “open process”. Once a den has reached the desired level thecarousel is rotated to position another den under the reactor discharge.The process is then repeated. Once the second den is filled, thecarousel rotates again to position a fresh den under the reactordischarge.

[0107] The filled dens rotate on the carousel, with the reactioncontinuing within each den. Effectively the static dens of the “openprocess” are replaced with rotating bins.

[0108] The number of dens on the carousel is selected so that thereaction has time to proceed to completion before the den needs to beemptied and refilled. The larger the number of dens, the longer theperiod prior to refilling, and the greater the flexibility in modifyingthe plant throughput. The material remains in the rotating dens for aperiod of approximately 10-120 minutes, but usually around 20-30minutes.

[0109] At the end of the cycle, the full den reaches the last positionon the carousel before refilling. At this point a hydraulic arm tips theden, discharging the contents into a lined bin. The lined bin optionallycontains a rotary cutter to break up any lumps formed.

[0110] However the tipping operation and top of the bin remain enclosedwithin the fume control system. The bin discharges the contents onto anenclosed handling system, such as a shrouded belt conveyor. Thishandling system takes the product to a maturation heap or for furtherprocessing.

[0111] Each den and the discharge bin are made from steel or a suitablecomposite material. There are no sharp corners within the dens or bin,and each vessel is lined with a suitable, slippery,temperature-resistant material, such as PTFE or similar. The lining isselected to withstand the maximum temperature reached during thereaction, but still release the product easily on tipping.

[0112] The den size is designed typically to take 500-1000 kg of theproduct and there are typically about 12-16 bins on the carousel.

[0113] In an embodiment of the invention, the reactor is fed byappropriate continuous handling/weighing systems. The material isaccumulated in the reactor and discharged before the product thickenssignificantly. In another embodiment a continuous high-speed mixer witha short retention time is employed. A flap eliminates spillage duringthe rotation of the dens.

Enclosed Continuous Process

[0114] In the “enclosed continuous process” the same principles arefollowed as in both the open and the totally enclosed batch processes.As before, the key principles are: rapid weighing of the raw materials,rapid mixing, and rapid transfer to a reaction zone.

[0115] In different embodiments of the enclosed continuous process, theraw materials are weighed out either on a batch basis, as described forthe totally enclosed batch process above, or on a continuous basis usingcontinuous mass measuring apparatus and a high-speed continuous mixerwith a short residence time.

[0116] Effectively a moving den in the form of an enclosed endless beltreplaces the static or moving dens of the batch processes. The enclosedbelt is fitted with extraction ports to extract steam and fumes to afume handling system.

[0117] The enclosed continuous process is initiated by weighing the rawmaterials and discharging the weighed materials sequentially into thereactor as described in the previous examples. The charging time of thereactor is essentially the same as that of the batch processes describedabove. Again, in different embodiments, the reactor is dischargedbatch-wise or continuously. The mixing time in the reactor is alsoessentially the same as that of the batch processes described above.

[0118] The reacting magma flows down a chute onto a suitabletemperature-resistant endless flexible belt which is typically of amaterial such as thin stainless steel or PTFE. The belt speed can bevaried to control the depth of material on the belt. In differentembodiments, the belt is troughed (forming sidewalls and individualcompartments) or folded on itself in the shape of a shallow “U”. Thebelt slopes slightly away from the feed end towards the productdischarge end so that reaction mixture or magma flows away from the feedend as long as it is still fluid.

[0119] The walls of the belt (or pockets of the troughed conveyor) formthe bottom and sidewalls of the moving belt, eliminating spillage. Aroof with at least one steam/fume extraction point runs the entirelength of the conveyor so that steam and fumes produced during thereaction are removed to the fume handling system.

[0120] The reaction between the acid and unslaked lime proceedsessentially to completion on the belt. At a distance down the belt(determined by the plant capacity and speed of the belt) the materialthickens, and acts as a dam wall against the further flow of fluidreacting magma.

[0121] At a point past the area where the material has hardened (nearerto the product discharge end), the belt flattens out. This is achievedby the belt running over a flat sloping plate or wide conveyor idler.

[0122] The flattening out of the product on the belt causes the heap ofmaterial travelling on the belt to pull apart, so that cracks andfissures are formed. The cracking up and crumbling of the surface allowssteam and trapped fumes to escape. As this is taking place within thefume extraction hood, no fumes escape to the surrounding working area.The flattening out of the belt also results in the belt floor widening.This causes material to be dislodged from the sidewalls as they flattenout and reduces the chance of material adhering to the fume hood. At theend of the belt, the material falls into a lined bin equipped with arotary cutter and rotating brushes clean the belt as it returns to thefeed end. The material freed by the brushes drops into the bin. The topof the bin is enclosed within the fume extraction hood.

[0123] The bin discharges onto a suitable enclosed transport system,which moves the product to a maturation heap or for further processing.

[0124] Typically the product depth or thickness on the belt will bearound 10-90 cm across a belt approximately 2 m wide. The enclosedsystem is typically up to 25 m long with a roof height of up to 2 m. Thematerial remains on the belt for a period of from 10 to 120 minutes,depending on several factors (such as plant throughput and limereactivity). The product can reach temperatures in excess of 230° C.whilst moving through the den.

[0125] Prior art methods for the production of the calcium salt of2-hydroxy-4-(mercaptomethyl)butanoic acid of which the Applicant isaware include reacting the hydrolysis product of the correspondingnitrile with an aqueous lime slurry and reacting a basic calcium saltwith 2-hydroxy-4-(mercaptomethyl)butanoic acid in the presence of a“heel” of the dry product calcium salt. The heel is usually a low gradeform of the calcium salt containing between about 3 and 12% moisture. Inthe absence of a “heel”, it has been reported that the reaction massbecomes viscous and difficult to mix. In these prior art processes, the“heel” is generally provided by recycling a portion of the productcalcium salt back into the reaction mixture. For example, a prior artprocess known to the Applicant recycles 200 parts per hour out of 593parts fed into a reaction zone. In the continuous process of thisinvention, no recycling is required. The process of the inventionresults in substantial cost savings since the additional equipmentassociated with transferring, measuring and controlling the recyclestream is not required. Effectively, the difference is between a recycleprocess and a Continuous Stirred Tank Reactor (CSTR) process. In anembodiment of the continuous process of the invention, an initial bed ofthe calcium salt product serves to replace the “heel” of the prior art.In another embodiment, most of the reaction occurs away from the mixingzone and no “heel” or bed of calcium salt is required. Similarly, thebatch process of the invention requires no “heel”. It is an advantage ofthe invention illustrated that the method of the invention, whichinvolves a rapid mixing step followed by a rapid transfer step, producesa dry product containing less than 5% water, without the need to includea “heel” in the reaction mixture and without the viscosity problemreported in the prior art. An important feature of the invention is therapid mixing of the calcium salt and the acid which allows the mixtureto be transferred to the receiving vessel before it becomes too viscous.This, in turn, allows the heat build-up in the reaction mixture to driveoff moisture to produce a free-flowing product with a low water content,typically less than 5%, without the need for a further expensive dryingstep. If a lower water content is required, the product can optionallybe dried further.

[0126] The Applicant believes that the method of the invention willresult in the cheaper and more efficient production of the calcium saltof 2-hydroxy-4-(mercaptomethyl)butanoic acid than that described in theprior art.

1. A method of making calcium-2-hydroxy-4-(mercaptomethyl)butanoate, themethod including the steps of combining and mixing2-hydroxy-4-(mercaptomethyl)butanoic acid and a calcium salt selectedfrom calcium oxide and calcium hydroxide and water in a first reactionzone, the combining and mixing step being carried out over a firstperiod of 3-120 seconds to produce a reaction mixture in the firstreaction zone; transferring the resulting reaction mixture at the end ofthe first period from the first reaction zone to a second reaction zone,the transferring step being carried out over a second period of 3-60seconds; and allowing heat generated by reaction between the2-hydroxy-4-(mercaptomethyl)butanoic acid and the calcium salt in thesecond reaction zone to drive off sufficient water to produce a productmixture containing less than 5% (m/m) water.
 2. A method as claimed inclaim 1, in which the first period is 5-20 seconds.
 3. A method asclaimed in claim 2, in which the second period is 5-45 seconds.
 4. Amethod as claimed in claim 0.1, in which the2-hydroxy-4-(mercaptomethyl)butanoic acid is in the form of an aqueoussolution.
 5. A method as claimed in claim 4, in which the aqueoussolution has a concentration of 65-95% by mass of the2-hydroxy-4-(mercaptomethyl)butanoic acid.
 6. A method as claimed inclaim 5, in which the aqueous solution has a concentration of 70-90% bymass.
 7. A method as claimed in claim 4, in which the combining andmixing steps are conducted so that the reaction mixture contains 70-90%by mass of the aqueous solution, 10-30% by mass of the calcium salt and0-20% by mass of water.
 8. A method as claimed in claim 1, in which theheat generated is allowed to drive off sufficient water to produce aproduct mixture containing less than 4.5% water.
 9. A method as claimedin claim 1, in which the heat generated is allowed to drive offsufficient water to produce a product mixture containing less than 2%water.
 10. A method as claimed in claim 4, which includes the prior stepof warming the solution of 2-hydroxy-4-(mercaptomethyl)butanoic acid to50-130° C.
 11. A method as claimed in claim 4, which includes the priorstep of warming the solution of 2-hydroxy-4-(mercaptomethyl)butanoicacid to 70-85° C.
 12. A method as claimed in claim 4, in which thecalcium salt is calcium oxide and in which 55-65 kg of the aqueoussolution is combined with 9-18 kg of the calcium salt over a firstperiod of 3-10 seconds.
 13. A method as claimed in claim 12, in whichthe duration of the second period is 3-15 seconds.
 14. A method asclaimed in claim 4, in which 450-550 kg of the aqueous solution iscombined with 75-150 kg of the calcium salt over a first period of 3-30seconds.
 15. A method as claimed in claim 14, in which the duration ofthe second period is 3-20 seconds.
 16. A method as claimed in claim 1,which includes the further steps of successively combining and mixing aplurality of batches of the acid, calcium salt and water in the firstreaction zone to produce successive batches of the reaction mixture andsuccessively transferring each of the batches to the same secondreaction zone.
 17. A method as claimed in claim 16, in which a materialdosing system, controlled by a programmable logic controller, weighs outthe raw materials into the first reaction zone.
 18. A method of makingcalcium-2-hydroxy-4-(mercaptomethyl)butanoate, the method including thesteps of combining and mixing 2-hydroxy-4-(mercaptomethyl)butanoic acid,a calcium salt selected from calcium oxide and calcium hydroxide andwater in a first reaction zone to produce a reaction mixture in thefirst reaction zone; and continuously transferring the reaction mixturefrom the first reaction zone to a second reaction zone, the reactantsbeing added to the first reaction zone in successive batches and thereaction mixture being continuously removed from the first reaction zoneat a rate which is selected so that the residence time of the reactionmixture in the first reaction zone is 5-60 seconds; and allowing heatgenerated by reaction between the 2-hydroxy-4-(mercaptomethyl)butanoicacid and the calcium salt in the second reaction zone to drive offsufficient water to produce a product mixture containing less than 10%water.
 19. A method as claimed in claim 18, in which the heat is allowedto drive off sufficient water to produce a product mixture containingless than 5% water.
 20. A method as claimed in claim 18, in which therate of removal of the reaction mixture from the first reaction zone is500-30000 kg per hour.
 21. A continuous method of makingcalcium-2-hydroxy-4-(mercaptomethyl)butanoate, the method including thesteps of simultaneously feeding, into a reaction zone, an aqueoussolution of 2-hydroxy-4-(mercaptomethyl)butanoic acid, at a rate of500-30000 parts per hour, and a calcium salt selected from calcium oxideand calcium hydroxide, at a rate of 150-6200 parts per hour, to producea reaction mixture in the reaction zone; and transferring the reactionmixture from the reaction zone to a second zone at a rate which isselected so that the residence time of the reaction mixture in thereaction zone is sufficient to allow heat generated by reaction betweenthe 2-hydroxy-4-(mercaptomethyl) butanoic acid and the calcium salt inthe reaction zone to drive off sufficient water to produce a productmixture containing less than 5% water.
 22. A method as claimed in claim21, in which the rate is selected so that the residence time of thereaction mixture in the reaction zone is 5-30 minutes.
 23. A continuousmethod of making calcium-2-hydroxy-4-(mercaptomethyl)butanoate, themethod including the steps of simultaneously feeding, into a reactionzone, an aqueous solution of 2-hydroxy-4-(mercaptomethyl)butanoic acid,at a rate of approximately 500-30000 parts per hour, and a calcium saltselected from calcium oxide and calcium hydroxide, at a rate ofapproximately 150-6200 parts per hour, to produce a reaction mixture inthe reaction zone; and transferring the reaction mixture from thereaction zone to a second zone at a rate which is selected so that theresidence time of the reaction mixture in the reaction zone issufficient to initiate reaction between the2-hydroxy-4-(mercaptomethyl)butanoic acid and the calcium salt but notsufficient to drive off water from the reaction mixture and allowingheat generated by further reaction between the2-hydroxy-4-(mercaptomethyl)butanoic acid and the calcium salt in thesecond zone to drive off sufficient water to produce a product mixturecontaining less than 5% water.
 24. A method as claimed in claim 23, inwhich the residence time in the reaction zone is 5-60 seconds.
 25. Amethod as claimed in claim 24 in which the second zone is an endlessmoving belt.
 26. The calcium salt of2-hydroxy-4-(mercaptomethyl)butanoate prepared by a method as claimed inclaim
 1. 27. A method of making a salt of2-hydroxy-4-(mercaptomethyl)butanoate, the salt being a salt of a metalselected from magnesium, sodium and potassium and the method includingthe steps of combining and mixing 2-hydroxy-4-(mercaptomethyl)butanoicacid and a base selected from magnesium oxide, sodium hydroxide andpotassium hydroxide in water in a first reaction zone, the combining andmixing step being carried out over a first period of 3-120 seconds toproduce a reaction mixture in the first reaction zone; transferring theresulting reaction mixture at the end of the first period from the firstreaction zone to a second reaction zone, the transferring step beingcarried out over a second period of 3-60 seconds; and allowing heatgenerated by reaction between the 2-hydroxy-4-(mercaptomethyl)butanoicacid and the base in the second reaction zone to drive off sufficientwater to produce a product mixture containing less than 5% (m/m) water.28. A magnesium, sodium or potassium salt of2-hydroxy-4-(mercaptomethyl)butanoic acid prepared by a method asclaimed in claim 27.